1. Field of the Invention
The present invention relates generally to the field of communications networks and, more specifically, to networks which include both half-duplex communication channels and full duplex point-to-point communication channels.
2. Discussion of the Prior Art
Dramatic growth in the use of small computers and workstations has resulted in increased demand for local area networks (LANs) which are capable of serving dozens or hundreds of stations. A typical LAN includes a number of stations which are interconnected by a number of communication links. (The terms "link" and "channel" are used interchangeably herein.) A station may consist of a personal computer, workstation, bridge or any of a number of other information processing or storage devices. Some of the stations may be "half-duplex stations" meaning that they are capable of either transmitting or receiving information at any given time, but not both simultaneously. Other stations may be "full duplex stations" which are capable of transmitting or receiving information at any given time or both simultaneously. Similarly, some of the communication links may be "half-duplex links" which are capable of supporting only half-duplex communication, while other links are "full duplex links" which are capable of supporting both half-duplex and full duplex communication. Any two given stations may be effectively connected by a communication "path" which is (1) half-duplex along its entire length (consists entirely of half-duplex links), (2) partly half-duplex and partly full duplex, or (3) full duplex along its entire length.
The primary function of a LAN is to provide a communication channel or link through which a given station, possibly from a remote location, may communicate with one or more other stations. A given link within a LAN is characterized by a protocol which defines when and in what manner stations may transmit and receive information through that link. Each station using that link must operate in accordance with the protocol in order to communicate effectively with other stations and to avoid interfering with other stations. Thus, a typical protocol sets forth rules or conditions regarding timing, data formats and the like.
An example of a protocol which is widely used in business environments is set forth in ISO 8802-3:1989(E) and is sometimes referred to as "Ethernet." The protocol set forth in ISO 8802-3:1989(E) is an example of a type of protocol known as "carrier sense multiple access with collision detection" ("CSMA/CD"). As explained below, CSMA/CD is a "contention" protocol in which a plurality of stations contend for and share a single communication channel through which information is passed using a "packet switching" technique.
According to the CSMA/CD protocol set forth in ISO 8802-3:1989(E), each station is required, prior to transmitting any information through a communication channel, to first check the channel in order to determine whether any other station is transmitting at that time. If no other station is transmitting, then the station which checked the communication channel (subject to other requirements) is permitted to transmit its information. However, if another station is already transmitting, then the first station must wait a period of time and check the channel again. Thus, a mandatory requirement of the CSMA/CD protocol is that only one station may legitimately transmit information at any given time.
The requirement that each station check for activity on the communication channel prior to transmission represents one aspect of the "scheduling" that is provided by the CSMA/CD protocol. It is this scheduling which permits multiple stations to effectively communicate by sharing a single channel.
However, the scheduling provided by the CSMA/CD protocol represents a trade-off of resources or performance within the LAN. While scheduling allows a number of stations to effectively share a single communication channel over a period of time, this is accomplished at the expense of reduced bandwidth since only one station is allowed to transmit information at any given time. Specifically, because only one station may legitimately transmit information at a given time according to the CSMA/CD protocol (even though the transmitting and receiving stations and the link which connects them may all be capable of full duplex communication), only one-half the amount of information may be transferred per unit of time as compared to full duplex communication in which a station may simultaneously transmit and receive information.
Therefore, a major disadvantage of the CSMA/CD protocol is the mandatory requirement of half-duplex communication which effectively limits the bandwidth of a communication link to one-half the maximum possible bandwidth between two full duplex stations which are connected by a full duplex link.
In certain situations, even though a station has properly checked the channel prior to transmission and has determined that no other station is transmitting on the channel, a "collision" of two transmissions may nonetheless occur. For example, assume that one station has just started its transmission but, due to propagation delay, the transmitted information has not yet reached its intended destination. Simultaneously, a second station which is ready to transmit checks the channel and, unaware that the earlier (or another) transmission is still in transit, determines that no other station is transmitting and begins to transmit its own information. The information transmitted by the second station may then "collide" with the information transmitted by the first station. As a result of the collision, both of the transmitted messages may be prevented from reaching their intended destinations.
In order to recover from collisions, ISO 8802-3:1989(E) provides for a "collision detect" signal which accompanies each transmission. Each station monitors the collision detect signal and, if a collision occurs, the affected stations reattempt transmission until either successful or a maximum number of allowed attempts is reached without success.
For the collision detection technique to work properly, all transmitting stations must always have sufficient time to detect the occurrence of a collision. Consequently, the physical size of the LAN must be limited so that the propagation delay between any two stations does not exceed a defined maximum propagation delay. Observance of the maximum propagation delay time guarantees that a given transmission is either completed successfully or a collision is detected within a finite amount of time prior to completing the transmission.
To comply with the maximum propagation delay required by the CSMA/CD protocol, the maximum physical distance between any two stations must be limited. Generally, a link which operates using the CSMA/CD protocol is limited to a maximum distance of 2.8 to 4.5 kilometers. Such a short maximum distance between stations often prevents the use of such links for applications where remote stations are separated by relatively large distances.
Thus, another major disadvantage of the CSMA/CD protocol is the maximum propagation delay requirement which effectively limits the physical distance between stations and the distance covered by the LAN.